text:
Which FAA regulations require compliance with TCAS II Resolution Advisories?
I've always heard that complying with a TCAS II Resolution Advisory is a required action for pilots qualified and operating an equipped aircraft. I am not familiar with the regulatory basis for this, though. I'd like to get a bit more specific with the context. For aircraft operating under 14 CFR 91, is there a regulation that can be referenced?? For aircraft operating under 14 CFR 135, is there a regulation that can be referenced? And what about for 14 CFR 121?
I'd also like to know if there are exceptions to any regulatory requirements.
This answer mentions that aircraft must respond within 6 seconds. I believe the source is AC 120-55C but I'm not totally sure.

It's part of the FCOM and QRH from manufacturers. FAA ensures operators include this in their SOP. Airbus has replaced previous "should... unless..." by "must" follow RA. See page 25 of TCAS Recommendations

If not elsewhere, Ops Spec will probably be the regulating requirement outside of Part 91.

14 CFR 91 Appendix G Section 2 (g)

Traffic Alert and Collision Avoidance System (TCAS) Compatibility With RVSM Operations: All aircraft. After March 31, 2002, unless otherwise authorized by the Administrator, if you operate an aircraft that is equipped with TCAS II in RVSM airspace, it must be a TCAS II that meets TSO C-119b (Version 7.0), or a later version.

14 CFR 121.356

Effective January 1, 2005, any airplane you operate under this part must be equipped and operated according to the following table...

14 CFR 135.180

Unless otherwise authorized by the Administrator, after December 31, 1995, no person may operate a turbine powered airplane that has a passenger seat configuration, excluding any pilot seat, of 10 to 30 seats unless it is equipped with an approved traffic alert and collision avoidance system. If a TCAS II system is installed, it must be capable of coordinating with TCAS units that meet TSO C-119.

TSO C-119
Also see AC-120-55 for information about operational use and required training for parts 91, 121, and 135. Specifically Section 12 "TCAS Operational Use" outlines how the crew is required to respond to TA's and RA's. It is a few pages long so not appropriate to duplicate here.

That refers to fitment -- not TCAS RAs taking priority over everything else (save flying into the ground)

@UnrecognizedFallingObject See 135.180(b)(1), which outlines the flight ops manuals. These manuals are approved by the FAA, which is outlined in AC-120-55 for parts 91, 121, and 135 operations.

How would that work for aircraft which have TCAS as a supplemental fitment? (STCed in, basically) Especially if the operator is operating under straight part 91...

If the operator is using TCAS, even as a supplemental fitment, the crew is required to receive training and operate as per AC-120-55 #12 "TCAS Operational Use".

I think AC-120-55 section 12 is the answer the querent wants then

This answer does not answer the question. Question asks about a requirement for crew to respond to RAs, answer merely addresses requirements for aircraft equipment.

@RonBeyer ACs are advisory, not regulatory. The OP is asking about regulations, not advisories.

@UnrecognizedFallingObject Section12 of that AC repeatedly says should, and is not regulatory in nature anyway.  This doesn't answer the question which is looking for a requirement to follow the advisories.

The requirement to respond to the TCAS II resolution advisory is somewhat indirect.  one key fact is that any TCAS installation has to be approved:

14 CFR §91.221 Traffic alert and collision avoidance system equipment and use.
(a) All airspace: U.S.-registered civil aircraft. Any traffic alert and collision avoidance system installed in a U.S.-registered civil aircraft must be approved by the Administrator.
(b) Traffic alert and collision avoidance system, operation required. Each person operating an aircraft equipped with an operable traffic alert and collision avoidance system shall have that system on and operating.

Key points here:  1. The installation must be approved.  2. If it's installed, it has to be used.
From Part 121:

14 CFR §121.356 Collision avoidance system.
  Effective January 1, 2005, any airplane you operate under this part must be equipped and operated according to the following table: {Detail omitted}

This says anyone under Part 121 must have A TCAS installed and operating.
If you go back to Part 21, you can find that the installation approval requires a Type Certificate, Supplemental Type Certificate, or an Amended Type Certificate.
Part of getting that installation approval is a requirement for Airplane Flight Manual (AFM) or an AFM Supplement.  With regards to TCAS, the airworthiness approval is addressed by

AC 20-151B Airworthiness Approval of Traffic Alert and Collision Avoidance Systems (TCAS II), Versions 7.0 & 7.1 and Associated Mode S Transponders

Chapter 5 and Appendix A cover the requirements for the AFMS for TCAS.
While the AC is advisory, it is spelling out a means of complying with the rules, which in Part 23 (small A/C) and Part 25 (air transport) require a flight manual.
Elsewhere:

14 CFR §91.9 Civil aircraft flight manual, marking, and placard requirements.

requires operation in accordance with the limitations of the AFM.
In Part 121, airlines operate in accordance with their Op Spec, which will include similar rules.
And there is specific wording to address clearances and response to TCAS:

14 CFR §91.123 Compliance with ATC clearances and instructions.
  (a) When an ATC clearance has been obtained, no pilot in command may deviate from that clearance unless an amended clearance is obtained, an emergency exists, or the deviation is in response to a traffic alert and collision avoidance system resolution advisory. However, except in Class A airspace, a pilot may cancel an IFR flight plan if the operation is being conducted in VFR weather conditions. When a pilot is uncertain of an ATC clearance, that pilot shall immediately request clarification from ATC.
  (b) Except in an emergency, no person may operate an aircraft contrary to an ATC instruction in an area in which air traffic control is exercised.
  (c) Each pilot in command who, in an emergency, or in response to a traffic alert and collision avoidance system resolution advisory, deviates from an ATC clearance or instruction shall notify ATC of that deviation as soon as possible.

This basically acknowledges that pilots will respond to an RA and that they should report it as soon as they can after the fact.


text:
What are the criteria for conceptual design of thrust reverser in the use of braking?
I'm doing a project: Study of aircraft braking and pre-sizing of associated subsystems. We tried to find criteria for the reverser in the braking phase. I read FAR 25, but I did not find it.
Is there a criterion such as how many percent of the braking force must be applied by the reverser in certification documents?

You won't find it because aircraft are supposed to stop with in-op reversers, and are not allowed to use them during certification. Reversers are just for convenience.

As @John's answer alluded to, the rules for actual landing distance (25.125) and the accelerate-stop distance on dry runway (25.109) effectively prevent a manufacturer from taking credit for thrust reverser (T/R) when demonstrating/calculating for these distances. However, 25.109 does allow a manufacturer to take credit for T/R when dealing with a wet runway. This means that having a (good) T/R gives an operator more flexibility when dealing with adverse weather conditions, and this translates to more money saved (vs sitting on the tarmac).
Therefore, just like any other performance numbers, the performance of T/R is subject to market objectives. There is no regulatory requirement on how powerful the T/R has to be at max reverse.
That being said, this NASA memo has some helpful information on why airlines want thrust reverser at all. The following two graphs are cited from this source, which outline the percentage of work done by the respective braking systems for a typical jetliner on dry and wet runways. You can perhaps back-calculate the thrust of T/R from your design point:

Other factors to consider for design:

Foreign object debris ingestion from T/R deployment, especially at lower speed
Compressor stall from hot reversed gas, especially at low speed
Inadvertent deployment of thrust reverser in-flight. For most airplanes, this would be catastrophic.
Controllability in single T/R deployment on ground
For rear mounted engines, aerodynamic interference with rudder effectiveness

I'm confused by the Icy runway graph. Is that saying that shortly after 4000 ft the reversers & inlet momentum suddenly lose all effectiveness in braking and that the total overall braking work suddenly drops off?

@FreeMan The left and right are not connected. The first portion of the right graph isn't shown (or consider it's hidden beneath the first graph).

Ah, that makes more sense.

Thrust reversers are not accounted for in performance criteria for accelerate/stop and landing distance values, so there aren't really any cert requirements for what you would think of as minimum reverser braking performance.  Cert requirements mostly focus on controlability during landing/reject with asymmetric reverse thrust, and the safety related aspects of (preventing) reverser function in flight, that sort of thing.
In terms of stopping performance, their use is a performance "bonus" and the pilot has to option to leave them stowed, unstow them but leave them at idle, or use any reverse thrust amount between idle reverse and max reverse desired during braking on any landing or reject.

For wet/contaminated runway, however, manufacturers are allowed to take credit for T/R for performance.

Good point.  I'm trying to recall if the CRJ's QRH had reverser/no reverser data for contam runways but no longer have access to those sorts of documents having been retired several years.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
What causes a plane to lose its roll momentum
I am currently developing a flight simulator but there is one force I can't figure out how to simulate using real world physics. When the pilot pushes the stick to the right, the plane will reach a roll rate that it will keep; and when the stick comes back to its neutral position, the plane will stop rolling, more or less quickly.
Is there a formula, or an approximation that explains how the plane will stop its roll? On what does it depend? Apart from wing area, does airspeed and lift impact this? Or does it depends on angular velocity?
I need a formula that I can compute to slow my roll down.

If you were using "real world physics", things like this would just happen, and you'd be looking at your equations trying to understand why roll (rate) stops. In truth, you seem to be trying to simulate phenomena (roll damping in this case) using pre-existing knowledge about flight mechanics. This is how it's usually done, but this is a big subject in itself. You might now incorporate roll damping using given answers, but you'll invariably miss something else equally important. You need a good book on flight dynamics.

Do NOT underestimate the roll resistance of wings. I want you to try this experiment to get an intuitive feel of how much wings resist rolls: get yourself a rubber powered propeller (actually, even a battery powered one works but will run a long time). Install the prop onto a stick, wind it up and release it - you will see the stick roll due to the torque from the prop. Now add a piece of card/sheet foam to the stick to make small wings and repeat - you will see the roll rate reduced. Now make the wings longer and repeat. With long enough wings there will be hardly any roll

Thank you both for your answers.
I finally succeed to implement roll daming naturally by taking in account the lift of each wing independently. Now with the AOA of each wing being different during the roll, the roll tends to stop when the stick is released.

The section about damping of how it flies can help you understand related concepts and mechanisms.

What you want is the roll constant $\text{T}_R$. This is basically one of the characteristics wich determines the equations of motion of an aircraft. It gives the slope of the roll speed increase over time with full aileron deflection and an ideally stiff wing, and equally the rate of decrease once the ailerons are set to neutral during a rolling maneuver.

With p the dimensionless roll rate, m the aircraft mass, i$_x$ its radius of the momentum of inertia about the roll axis, S its wing surface, b the wing span, v the flight speed, $\rho$ the density of air and $\text{c}_{lp}$ the roll damping coefficient, the formula is
$$T_R = \frac{2\cdot m\cdot\left(\frac{2\cdot i_x}{b}\right)^2}{\rho\cdot v\cdot S\cdot c_{lp}}$$
When the pilot moves the stick, the aircraft will accelerate into the roll but the acceleration will diminish with the square of roll speed until an asymptotic value is reached. This acceleration grows with increasing air density, flight speed, roll damping coefficient and lower wing loading and smaller square of the ratio of roll inertia over wing span. Same in reverse: Stopping will be fastest under the same conditions.
For the roll damping coefficient, use this approximation for wings with an aspect ratio AR larger than 4:
$$c_{lp} = -\frac{1}{4}\cdot\frac{\pi\cdot AR}{\sqrt{\frac{AR^2}{4}+4}+2}$$
Since this is a damping coefficient, it makes sense to be negative.
The asymptotic value is reached when the propelling moment from aileron deflection equals the retarding roll damping:
$$c_{l\xi} \cdot \frac{\xi_l - \xi_r}{2} = -c_{lp} \cdot \frac{\omega_x \cdot b}{2\cdot v_\infty} =  -c_{lp} \cdot p$$
For an explanation of this equation and all the terms used please see this answer.
Please note that this is all only valid for a stiff airframe. Increasing dynamic pressure reduces aileron effectiveness because the wing will warp when ailerons are deflected. Assume a linear decrease with dynamic pressure until only a fraction of the ideal roll acceleration remains at top speed and low level flight.

Now you ask about roll damping torque and that cannot be explained in the comments. Look at the last equation – it is already there, albeit dimensionless. To get from there to an actual torque, multiply by wing area, semispan and dynamic pressure:
$$T=\frac{\rho}{2}\cdot v^2_\infty\cdot S\cdot\frac{b}{2}\cdot c_{lp}\cdot p = \frac{\rho}{8}\cdot v_\infty\cdot S\cdot b^2\cdot c_{lp}\cdot\omega_x$$
with $\omega_x$ the actual angular speed in rad/s. Do the unit check - this is actually a torsion moment [Nm]. Please note that I used the reference length for lateral moments used in Germany; the US uses the full span instead of the semispan. So make sure you check which reference length your sources use!

What you gave me is PERFECT. I knew it existed equations I can fit into my simulation and see what I get.
And there is one thing I dont understand, the speed of the plane is involved here but I don't see any angle of attack. Is the AOA considered of 0 for the equation ?

But actually what I am searching for is the equation of the roll damping torque. Something I could apply constantly and that would change with speed, AOA, etc.
Because the roll damping coefficient and the roll constant does not describe what happens regarding those parameters.
For example, what happens when the airplane is rolling and suddenly stalls ? With 0 airpseed, how will the roll rate vary ? I guess a plane will roll longer until it stops if put on a rotating axis on the ground, rather than flying 200km/h through the air.
I don't know if you see what I mean ?

@Anselme: Roll damping is independent of AoA. Almost. If the aircraft stalls, things get nonlinear and very individual for different designs. Roll damping can even become positive, so the aircraft accelerates into a roll without aileron command. That's quite scary when it happens at low speed and can easily ruin your day.

@PeterKämpf That's a "snap roll", "wing drop" or asymmetric stall, isn't it? If the aircraft stalls while in a sideslip, then the downwind wing may stall before, or more severely than, the upwind wing, producing a sudden roll away from the relative wind. (You know how all this works, of course; I'm asking if my understanding is correct.)

@TerranSwett: Yes, your understanding is correct.

Your question is an essence a question about aerodynamic damping in the roll axis.  As the aircraft rolls, the rolling motion increases the angle-of-attack of the descending wing and decreases the angle-of-attack of the rising wing, eventually causing the lift created by each wing to be equal despite the deflected ailerons.  At this point roll torque is zero and the roll rate can no longer increase.  If the stick is brought back to center, the roll torque from aerodynamic damping will soon cause the rate will soon drop to zero, or to near zero.  (Yes, it is possible for an airplane to tend to roll toward a steeper or shallower bank angle with the stick centered, but the roll rate is much lower than we see with the stick highly deflected to one side.)
Read more about in this section of John S. Denker's excellent "See How It Flies" website.
For the same wing area, roll damping will be greater with a large wingspan than with a small one.  This is why airplanes with smaller wingspans generally achieve higher roll rates than aircraft with larger wingspans.
For the same external physical shape, a high moment of inertia in the roll axis (due for example to having the wings full of fuel or loaded with bombs) will cause the roll damping effect to take more time to bring the roll rate to zero (or near zero) after centering the stick.
To get actual formulae, you are probably going to have take a deep dive into some textbooks on flight dynamics or aircraft design.
Rolling also creates an adverse yaw torque, even with ailerons centered, which creates sideslip which interacts with any dihedral or sweep that is present to slow the roll rate.  See this section of the "See How It Flies" website.  But that's probably a finer-scale effect than you are looking to take into account.  (The complex subject of the roll torque generated by the interaction between sideslip and the aircraft's 3-dimensional geometry has been touched on in several different questions on the ASE site.)
You'll find plenty of other content in the "See How It Flies" website that should help you with your project as well.

Thank you for your help.
So if I understand well, the downgoing wing is producing more lift that the upgoing one, causing a torque opposing the rolling motion of the plane ? Is this what is responsible of roll damping ?
If thats right then I now know how to simulate roll damping as I already have functions to calculate wing lift with AOA.
Can you confirm I understood well ?
Thank you

Yes--sounds like you understand.

Ok, thank you very much for your help, I appreciated it

@quiet flyer I assume the second "increases" on the second line should be "decrease"?

@JohnK -- good catch

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Jokes only pilots would get in Airplane
In the 1980 comedy movie Airplane!  Are there some jokes that only pilots would get?  I always thought there was sight gag with the runway when they are taking off.  The runway look likes it has exit ramp like an interstate highway.

"Airplane!" (1980) includes jokes that only James Hong gets. That film is such a dense farce that even if you see something in it that is not funny, you can be confident that somebody somewhere is laughing at it.

I always wondered how many non-pilots got the piston-engine-sound-effect-in-a-jet gag

That one was done as a big f**k you to the Paramount Execs by the Zucker Bros. who originally wanted to movie to take place on a propeller powered airplane as was in the movie Zero Hour which Airplane! satirized.  The executives demanded the film take place aboard a jet, but the directors got the final laugh adding in the propeller sound effects.

Not really.  That movie had more gags associated with everyday objects eg Capt. Oveur initiates taxiing the 707 by shifting it into gear, like a car, etc.  I don’t really recall any aviation specific elements - aside from that it took place on a commercial airline flight - that only pilots or someone with a deep knowledge of aviation would get.  One aviation item which gets a little overlooked was names of the flight crew - Captain Clarence Oveur (Clearance, Over), FO Roger Murdoch, and FE Victor Boston, being plays on common radio terms “Roger, Roger.  What’s out vector, Victor?”  Another one I recall was a minor scene between Lloyd Bridges and Robert Stack where they argue the pros and cons of letting Stryker do a normal landing vs foaming the runway and letting him land wheels up.  Of course what made that scene funny was the slapstick use of a water cooler.  Bridges had another scene with Stack where Stack remarks “That’s impossible!  They’re on instruments!” which cuts to a scene with Stryker, Elaine, and Dr Rumak playing jazz using musical instruments in the cockpit.  Most of the film’s jokes had to be more relatable to the man on the street’s concept of air travel eg “the White Zone, Red Zone” recordings they used to play, being pestered by religious irritants for money in terminals, etc.  I don’t think Airplane! would’ve have had nearly the appeal if it’s humor was based on a deep knowledge of aviation.

bravo, carlo!!!


text:
Do pilots still not need additional simulator training to fly the 737 MAX?
Boeing added the MCAS system to the 737 MAX so pilots didn't have to go through expensive simulator training to get used to the changed flight profile caused by the engine redesign. Do pilots still not need that additional training now that the software has hopefully been fixed and the plane is cleared to fly again?

One of the conditions for allowing the 737 MAX to operate again, was additional simulator training for all pilots. In principle, each aviation authority can make their own rules, but the 4 big western authorities worked together to establish common criteria for the MAX's return to service:

EASA, and regulators in Canada and Brazil, worked closely with the FAA and Boeing throughout the last 20 months to return the plane safely to operations.

(EASA declares Boeing 737 MAX safe to return to service in Europe)
In Europe, the new Airworthiness Directive (AD) 2021-0039R2 requires simulator training for all MAX pilots:

[E]very individual 737 MAX pilot needs to undergo a once-off special training, including simulator training, to ensure that they are fully familiar with the redesigned 737 MAX and trained to handle specific scenarios which may arise in flight. This will be reinforced by recurrent training to ensure the knowledge is kept fresh.
[...]
In summary, the EASA Airworthiness Directive mandates the following main actions:

Software updates for the flight control computer, including the MCAS
Software updates to display an alert in case of disagreement between the two AoA sensors
Physical separation of wires routed from the cockpit to the stabiliser trim motor
Updates to flight manuals: operational limitations and improved procedures to equip pilots to understand and manage all relevant failure scenarios
Mandatory training for all 737 MAX pilots before they fly the plane again, and updates of the initial and recurrent training of pilots on the MAX
Tests of systems including the AoA sensor system
An operational readiness flight, without passengers, before commercial usage of each aircraft to ensure that all design changes have been correctly implemented and the aircraft successfully and safely brought out of its long period of storage.

(EASA declares Boeing 737 MAX safe to return to service in Europe, emphasis mine)


text:
Is it required to be an ATP and work for an airline to be a Captain?
I'm just curious if the ATP certificate and working for an airline are prerequisites to being a Captain, or could you have such a title as a PPL or a CPL?

I just can't help but think of the Captain Morgan commercial

The title "Captain" is not a title bestowed by the FAA.  The airlines themselves bestow that title for an ATP rated pilot who has seniority and is shown to be proficient for the airplane flown.  A pilot could be deemed "captain" for one flight and "first officer" for another.  In the US a pilot must have an ATP certificate to be a "first officer" or "captain".
Could somebody bestow that title on a pilot with only a PPL or CPL?  I suppose so, but calling somebody Captain doesn't really say anything about their credentials.

I don't think this is true in every case.

@DJClayworth only potential active airline pilot I can think of who doesn't need an ATP might be one who does ferry flights and probably cargo runs exclusively, never carries passengers. Which would probably be one who's working for say DHL or Atlas, which most people wouldn't call airlines though technically they are.

The key phrase, legally speaking, is "pilot in command".  On a multi-crew aircraft the PIC is normally given the honorific title of Captain, but what really matters is the legal status of PIC where an aircraft legally requires more than one crew.  In general to be PIC of a multi-crew a/c you need an ATPL.  Operators may operate single-pilot aircraft like Caravans as if they were multi-crew with two pilots and call the PIC Capt, but again that's just an honorific title and in that case the PIC can be just a CPL.
Like DLH says, a PPL or CPL can call themselves anything they want; Captain, Great Exalted Poobah, Zontar The Munificent, whatever.

+1  Every so often I fly my spamcan into an airport, go to pay my landing fees and get asked "who is the captain?", usually because there's a box on a form.  Ain't no captains here, just a weekend fairweather PPL, but if they want to call me that...

Hmm, I would have liked Great Exalted Poobah. I used to know a captain, who, when establishing contact with the groundcrew would start by saying, "Sky God to Ground Scum." I always wondered if there were some on ground crews who took offense.

@Terry "Ground Scum" - sounds like a great way to get put at the end of the holding pattern. And to continually get bumped back there when new flights arrive...

@FreeMan Well, the ground crew I was referring to was the marshalling people on the interphone hooked into the nosegear port, so it wasn't ATC. And he palled around with them a lot, so they knew he didn't really mean it, but still I think it was hurtful to some.

@Terry ah! That makes much more sense! ;)

"In general to be PIC of a multi-crew a/c you need an ATPL." How does John Travolta get around this?

He flies his big jets alone?  Private a/c in the US are a different story.

When Pan-Am created the first modern airline, most passengers of that era were familiar with the practice of ships having a naval-uniformed "Captain" and/or "First Officer" that greeted them on boarding.  Pan-Am decided to copy that practice by dressing the pilots of their "flying boats" dress in the same uniforms and use the same titles, in hopes this would set themselves apart from barnstormers (who frequently killed themselves and their passengers) as a "professional" outfit and put passengers at ease.  Other airlines soon copied the practice, and it has survived to this day despite the move to land-based planes long ago.  However, the titles have absolutely no legal status, unlike with ships.
Legally, what matters for flying is the "Pilot in Command" and, for aircraft that require one, the "Second in Command".  The type of pilot license required for each varies by country, type/size of aircraft, and whether the flight is scheduled, charter or private.  For example, a student pilot can be "Pilot in Command" of a single-engine trainer as long as they are not carrying passengers and have the correct endorsements from their instructor.

correct, and to make matters more complicated there's the Pilot Flying and Pilot Not Flying, roles which don't have to correspond to the Pilot in Command and Second in Command roles (e.g. in a training aircraft the PF is usually the student, the PIC always the instructor).

There are a tiny number of people who are rich enough to fly what we think of as airliners for their own amusement. If they are not flying for profit they do not need ATP licenses, nor to work for an airline, and can be called "Captain" by any standard.


text:
Do aircraft engine intakes require air filters?
I see helicopters which have air filters in their engine intakes, but jet fighters and commercial turbojet airliners don't seem to have any filters in their air intakes.
Isn't it true that even tiny particles can damage the turbofan's blades, especially since they rotate at such high speed?
What if the aircraft has to fly through a dusty area?

Note that filters are not always the solution. For example, the Su-27 has closable vents on the top of its main two inlets where the main airflow entry can be closed and those two flaps opened in the case of a runway with excessive debris.

@Jihyun, MiG-29, not Su-27.

@Zeus thanks, sorry about the misinfo. So similar I get them confused all the time.

In general, if you look at military tactical transports, they are all fitted with turboprops, because they are less prone to FOD / dust damage

Dust does damage the blades but the effect is like sand blasting; it takes some time to cause structural damage. The turbine engines of airliners can take some dust (by design) but also get extensive maintenance regularly. And if they do get caught in a dust storm they should get out ASAP and get the engines inspected.
The biggest danger dust-wise is volcanic ash as that will adhere to the turbine blades behind the combustion chamber and potentially shut the engine down due to compressor stall. This happened with British Airways Flight 9 where all 4 engines failed. This is why a volcano can shut down Europe airspace.

Also, jet airplanes spend most of their time above the haze line, where the air is fairly clean (not much dust or other particulates, volcanos excepted of course). Helicopters usually fly near the surface in much dirtier air.

Layman comment: On Aircraft the bulk of the thrust comes from airflow that would have to pass through the turbine/bypass ... i.e. the 'filter' would impede the thrust. In a Helicopter, the thrust comes from the rotors, not the turbine. To make the analogy complete, the helicopter would have to put a filter over the rotors as well as the turbine!

The centripetal force in fan of a turbofan engine also causes larger debris to spun out and through the bypass duct. At least some engine also have another debris catcher after the low pressure turbine that ports debris that made it through the fan back to the bypass duct. Fine dust and sand can still make it through and can do a number to the HPT as the sand will melt and turn to glass then solidify in cooling passages. Cooling designers take this into account and try to design in pathways for the sand to get out of the cooling passage.

In addition to Rachet Freak answer I'll add that helicopter engines are subject to much dirtier air. Aircraft operate from runways which are relatively clean. Due to their speed, most dust and debris that is kicked up is behind the engine inlet so it is not sucked in.

Helicopters hover and land vertically and therefore their engines ingest air contaminated with kicked up dust. Especially when operating outside paved surfaces this results in considerable pollution. A filter is much more useful here.

How about MV-22 Osprey? It kicks up a lot of dust when vertical landing, but it doesn't appear to be equipped with filters.

@ashpool the MV-22 is equipped with Engine Air Particle Separators, fancy filters.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
What do pilots operating under FAA 14 CFR 91 have to do to fly in any of the Special Flight Rules Areas (SFRAs)?
What do pilots operating under FAA 14 CFR 91 have to do to fly in any of the Special Flight Rules Areas (SFRA). Does each SFRA have its own requirements? I know there's a question already about the Washington DC SFRA. Is there a master list of these areas with corresponding requirements?

The "master list" of special flight rules areas (SFRAs) in the United States is FAR 93 (14CFR93)) - aptly named "Special Air Traffic Rules".
It contains a list of areas with "Special Air Traffic Rules", including the requirements for operating in those areas.
Charts with SFRAs on them include a notice directing you to FAR 93 (and often other helpful information about the SFRA):

In addition to "Special Air Traffic Rules" some airspace with "special" rules may be defined by "Special Federal Aviation Regulations" (SFARs), found at the beginning of Part 91.
The only one I see in there currently is the Grand Canyon National Park SFAR.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Are records of certificate actions available?
Are there any online records that may indicate why a specific DPE may have lost qualifications recently?

Due to privacy concerns, certificate actions are not made publicly available without a formal request, and have to be made via an acceptable means.
The FAA has a document titled Information Provided by PRIA, FOIA, and the Privacy Act which goes into details about how information can be requested and what they will provide:

PRIA is basically a request made by a hiring air carrier, so you wouldn't be able to do that.
FOIA is the Freedom of Information Act, and the above document states:

Through FOIA, ANY enforcement action, whether still open or fully
  adjudicated and closed, can be reported.

The Privacy Act allows you to request your own records, or a third party company may request records, so neither of these would apply in your case either.

Also worth reading: FAA Order 1270.1 which is the FAA's internal process for dealing with FOIA requests. This order details what an "acceptable" request is, and how you should go about making it.

It probably won't answer your questions directly, but you can begin by looking through the FAA's Quarterly Enforcement Reports.
(You may be able to find more detailed information somewhere else on that same site. I'm not sure.)

Not what I was looking for, but interesting nonetheless, thanks!


text:
What materials are plane windows made of?
I've always wondered, what are those dual-layered (bonus: sometimes they have a little <2mm hole, why is it there?) windows commercial jets put next to their seats made of?

Related question: Why is there a hole in one of the layers of an airplane's window

Generally aircraft windows are made of what we colloquially call "plexiglass" of some kind (Lexan polycarbonate is common in light General Aviation aircraft, acrylic plastics are also used).  This material is light, relatively strong (not shatter-proof, but it'll take a moderate beating) and has decent optical properties.
Its major drawback is "crazing" -- hairline cracks on the surface of the material that form over time from dirt, stress, temperature, UV exposure, and the like.

Bonus answer: The little hole serves a couple of purposes, but the biggie is pressure relief/equalization. This gets discussed over on airliners.net a lot and they've covered it pretty thoroughly.
Basically the hole ensures that the cabin pressure is pushing against the outer (primary, usually thicker) sheet of plexiglass, which is plug-wedged into the fuselage structure and can't go anywhere.
Should the outer pane be damaged the hole will also theoretically bleed cabin pressure off at a controlled rate - though how successful that equalization is would depend on your altitude (and thus the pressure differential between the cabin and the outside air) and the degree of damage to the outer pane.

As for shattering, it only depends on thickness - fighter aircraft feature moderately bullet-proof canopies (not even talking about birdstrike) and all the huge show aquaria sport PMMA panes.

I wonder about the pressure relief thing, especially concerning pressure loss - not sure if this hits the spot (see also this).

@yankeekilo Yup - thickness and design (you can get layered polycarbonate or acrylics which basically function like kevlar) - if you've been to a bank in the US recently you've probably seen the wall of bullet-resistant plastic between you and the tellers.  Re: the hole, I'm not sure I completely buy the pressure transfer/relief thing, but I'm not sure I completely buy the condensation thing either (why not fill the middle chamber with dry nitrogen before sealing them together?) I suspect it's a little bit of both honestly, or possibly something else entirely. We need a jet A&P in here! :)

The cabin-side screen is just a flimsy, easily replaceable thing that is used to protect the actual window and add a bit of thermal insulation. It does not form any kind of hermetic cavity with the window, too, but sits inside the pressurized cabin (otherwise you´d get one hell of a draft). I´ll try to remember to ask a colleague for confirmation

@Yankeekilo: I remember seeing some documentary where Israeli air force tested it and showed that bird strike at supersonic speed can break fighter canopy (Israel is bottleneck on migratory paths with sea at one side and desert on the other, so they get really huge flocks of birds in spring and autumn).

@JanHudec A supersonic strike by a bird is (energy-wise) a really mean impact. That´s why I used "moderately".

@voretaq7: My local bank doesn't have anything of the sort, unless they've invented invisible and insubstantial plastic.

Windows are either made of acrylics (PMMA, brand name e.g. "Plexiglas") or polycarbonate (PC, "Lexan" or "Makrolon") materials.
The small hole is there to provide ventilation and enable removal of moisture/condensation, but the hole is only in the thin protective screen on the inside. The much thicker actual window that holds the pressure is (hopefully) not punctured.

Most windows (Boeing) are a triple layer of plexiglas -
Cabin air circulates between panes for defogging -
Except the windows found on cargo doors (electrical heat instead) -
Elasticity is the main reason why they are strong -
Windshields are different, must resist to bird strikes and be heated -
One inch or more in thickness and special optical corrections -
A windshield pane for a 727 was $10,000 in the 1980s...!
Note that some airplanes have two different Vmo -
Because of windshield bird strike resistance -
As an example, some Learjets -
Vmo (low altitude where birds are) is 305 KIAS -
Vmo (high altitude, above FL 140) is 358 KIAS -
I did fly as Learjet instructor in the early 1970s -
I was told the windshields of the 24/25 were "birdproof" -
Could resist a 4 lbs bird hitting the windshield at 350 mph -
I wondered if it could resist a 350 lbs turkey at 4 mph...?
ROFL -
One note also about a fire axe -
If you wish to make a hole to escape, do not try windows -
Windows are extremely resistant -
It is better to use a fire axe against metal (fuselage skin) -
Look at UK-CAA airplanes and "cut here" frames on side of fuselages -
Airfield firemen are knowledgeable about not trying to "break" windows -
And would open fuselages within the "cut here" frame indications -

There is a term called Tg, glass transition temperature. In PC this is around 150C. Therefore the mobility in the PC molecular chains at RT are unmobile, which in most amorphous polymers would cause them to be brittle. However due to the chemical bonding of PC this does not occur.
However, upon an impact, the stress energy resulting from a certain point will be transferred across many of the Polycarbonate chains. Induced impacts can cause large amounts of stress, especially at the speeds we are talking about. Therefore the holes are not only there as people have already said, for the equalisation of pressure differences, but it also a stress relieving point for the material.
Sources: I am a MEng Materials Engineer with experience in this field

This seems to focus on the reason for the hole, and not the type of material, which is what the question is asking.

In PC the cooling rate makes all the difference. Cool it too quickly or very slowly, and PC will become very brittle. The pressure relief hole has nothing to do with it, Mr. Materials Engineer.

@fooot The question explicitly asks what the hole is for, and the answer covers that. This is a problem with the question (and exactly why we don't like multi-part questions), not the answer.

However, this answer is obviously wrong. The small hole is in the middle layer of a three-layer side window. Any impact is going to be on the outer layer and, in any case, the side windows don't receive significant impacts. A bird strike to the windscreen is dangerous because of the 500mph+ closing speed. A bird strike to a side window would be no more dangerous than a bird flying into your house windows: the closing speed is just the bird's airspeed and the window is trying to get out of the way at 500mph perpendicular.


text:
Why does the hydraulic pump of retractable-gear GA aircraft run continuously throughout the gear cycle?
The POH of a Cessna T210N (which is a retractable-gear aircraft) states that the hydraulic pump will run 1 - 2 times per hour to restore system pressure bleed down, and whenever the gear up / down lever is repositioned for the whole gear cycle and 1 - 2 seconds more, to restore system pressure. Also, the emergency hand pump can only extend the landing gear, but not retract it. I think it's the same for other RG's (such as the C177RG, C172RG and C182RG).
Now to the question: First, why does the pump need to run all the time while the gear is moving ? Since there should be no pressure loss. Second, why can the emergency hand pump only extend the gear but not retract it - I thought all the emergency hand pump does is building pressure as the electrical hydraulic pump does, and gear position is selected by the gear lever.

Since there should be no pressure loss.

There is no fluid loss, but there most definitely is pressure loss.
Pressure is energy. More precisely, energy is pressure times volume. A hydraulic actuator works by letting a fluid flow from a high pressure reservoir to a low pressure one¹, extracting the energy from the high pressure to do useful work. So yes, there is a pressure loss², and since the energy required is more than what is stored in the high pressure reservoir, the pump must run during the gear motion.

I thought all the emergency hand pump does is building pressure as the electrical hydraulic pump does, and gear position is selected by the gear lever.

When extending the gear, gravity is doing most of the work, but when retracting it, you need to work against the gravity. Therefore you need more pressure to retract the gear than to extend it.
I don't know whether the hand pump is only connected to the extension piston as user3528438 mentions, but even if it is connected to both, it may not be able to reach the higher pressure needed for retraction.

¹ The low pressure reservoir is often the actuator itself—a piston that extends, increasing the volume of fluid it contains.
² Even if the pressure in the extending piston isn't much lower, pushing the fluid into it takes energy from the reservoir that the pump has to replenish by pumping more fluid there.

Additionally, if you've lost hydraulic pressure, you can fly with the gear down to either return to the airport you just took off from, or, if fuel allows, to your destination. Additionally, if you've got a hydraulic failure, you may have other issues to deal with than hand pumping your gear up. You can't land with the gear up. Well, not without generating a very large repair bill.

It comes down to the basics of hydraulics.
The pressure of the system is maintained by a pressure reservoir when the pump is not running, but the valve would leak a bit so the pressure "bleeds down" slowly.
However the pressure reservoir can in no way power the entire stroke of the actuator, hence the pump needs provide a continuous supply of pressure during the entire motion of the actuator.
To your last question, I think it's because of the failure mode of the system. If the hand pump is necessary, then that means either the mechanical pump has lost power or somewhere along the hydraulic lines is leaking. To defend against the latter you would want to minimize the length of hydraulic lines that the hand pump connects to, so the reasonable choice is to connect the hand pump directly to, and only to, the extending side of the actuator.

You mean an accumulator no?

This is the fundamental difference between pneumatic and hydraulic systems - hydraulic fluid is supposed to be incompressible, so for work to be done, there must be transfer of fluid - even a small increase in volume, due to a piston moving, will result in a big drop in pressure.

As user3528438 pointed out, the reservoir seldom has enough capacity to run the systems it is feeding. It's purpose is more like being a dampener, as the pump would otherwise produce quite spikey pressure waves across the system. Also maintaining the pressure in the system will help keep air (or impurities) in the system.
A reservoir large enough to cycle gears would be quite heavy, as required pressure is not exactly very small.


text:
What do professional commercial pilots do to counteract boredom?
With the eyes of a layman, it seems to be a wonderful thing to fly a big airplane between continents. From an initial perspective anyway.
During a transatlantic fight, pilots can see the water of the ocean, and the never changing readouts in the cockpit. Years long. Decades long. Only the water, the clouds, the sky, the sun and the sensor readings.
Until they retire!
Ok, maybe they could get a laptop with them and play on that, but I think it is probably against the rules.
Isn't it something like a prison for life? How pilots deal with the increasing boredom?


text:
How comes lift is less than weight on take-off?
If $$lift=weight * cos(angle)$$ this means that lift is less than weight during takeoff.
Could someone please explain me why it is so

where did you hear  that lift>weight during takeoff?

Why does the figure say L=W but also L=W*cos(α)? Anyway, the rest of the weight in the cos(α) case is supported by thrust.

must be an old book - the plane looks like the classic de Havilland Comet. Also, this airplane should fly straight, because $\alpha$ is the angle of attack, not the flight path angle. That would be $\gamma$.

Indeed, what puzzles a lot of people is that excess lift (excess as in more than we need for weight) it not what makes an aircraft climb.
Of course, a take-off is a dynamic situation which doesn't really call for a balance of forces and momentum, so the situation is a bit more complicated to explain, but let's try a thought:
Imagine it this way: If lift > weight you will accelerate 'up-'wards (ie. balloon style). Air will come not only from the front moving backwards, but also from above moving below. The relative air moving past your wing is now coming from front and slightly above. This decreases your angle of attack and thus decreases lift, usually up to the point where lift = weight again.
If lift< weight, the opposite happens (ie. accelerate downwards) and air will move from below the wing towards up. This increases the angle of attack, increases the lift, and again lift will match the weight.
It's basically a statically stable system, where the only really stable position ,if you think about, is when lift equals weight (in straight-level flight, mind you. For climbing and descending you have the formula stated in your question).
If you want to climb, increasing lift is not the way to go !

If you want to continue climb/descent/level flight, increasing lift is not the way to go.

Wing lift is not the only force with an upward component during climb - thrust will make up the difference, which is (1-cos$\gamma$)$\cdot$W.


text:
What are the general post flight sequence checks and do they start automatically? If so, when?
What post flight checks are performed? Are they automatically or pilot initiated. If automatic, what are the possible conditions?
For example, one of the conditions to start the postflight checks might be "aircraft wheel speed below 30-40 knots".

All checklists are initiated by either the pilot flying (PF) when between takeoff and touchdown, and the captain (CA) on the ground.  In addition to checklists is the concept of flows, which are a muscle memory sequence of buttons to press / switches to flip / things to touch that are performed by the appropriate pilot before the checklist is called for.  The flows will happen automatically when the pilot responsible knows they should happen and the checklist will be called for when the responsible pilot is ready for it.
For example, when landing, the PF will perform the landing, the deceleration and sometimes the high speed turnoff before control is transferred to the CA for taxi.  Once the CA is taxiing and the airplane is cleared of the active runway, the FO will automatically start the after landing flow and call ground control.  Once taxi is underway, the CA will call for the after landing checklist.
Once parked at the gate, both pilots will do their portion of the parking flow and then the CA will call for the parking checklist.
Finally, if no crew is waiting to receive the aircraft and you are not keeping the aircraft then the FO will perform the postflight inspection and the pilots will perform the terminating checklist.  If you are keeping the plane, the FO instead does a combined post/preflight inspection and begin normal pre-flight actions.
While these actions are all started by humans, the closest thing to automatic actions are the flows.  To summarize the above, with general criteria for starting flows:

Landed, clear of the active runway, CA has the plane -- perform after landing flow
Parked at the gate -- perform parking flow.

Each of these flows are backed up by checklists called for the CA.

After landing checks

These checks are designed to take the aircraft from a landing configuration to a ground/taxi configuration shortly after exiting the runway.  In general, you may see things like:

Retract flaps / spoilers
Transponder to 1200 / OFF / GND
Landing light on, strobe lights off

Shutdown checks

These checks are designed to configure the aircraft for shutdown once ground movement has stopped.  You'll see things like this:

Heater / air conditioner OFF
Magnetos DEAD CUT CHECK
Avionics MASTER OFF

Postflight checks

These checks are used once the aircraft has been shut down to make sure that it's secure or ready to be handed over to the next crew.  You'll see things like the following:

Flight log COMPLETE
Cabin CLEAN
Tie-downs SECURE

All of these checklists are pilot-initiated.


text:
On commercial jets, is asymmetric thrust used as part of normal operations?
I know that when manoeuvring on the ground, asymmetric thrust is often used, for example to assist in turns when taxiing.
Are there other circumstances in normal operations which might call for the use of deliberate asymmetric thrust in manoeuvring?
I am specifically not asking about emergency or other abnormal ops, flight testing or auto-throttle operation.  I could rephrase the question as "in day-to-day ops of flying an multi-engine commercial jet, is asymmetric thrust ever used to manoeuvre the aircraft when not on the ground?".

I assume that "engine out" does not count as "normal operations".

@FreeMan Correct. I'm specifically not asking about emergency or abnormal ops.

Would "Normal Testing Operations" count?

@SMSvonderTann No.  I am specifically interested in the day to day "just doing my thing" of flying commercial multi-engine jets.  Thanks.

This paper from Boeing sheds a light on the subject:

[A]n intentional engine throttle up or down could create a desired yawing moment followed by a desired rolling moment. Using asymmetric thrust to control roll is not precise because of the lag time associated with engine spool-up or spool-down and should be avoided unless no other means of roll control are available.

While the paper is admittedly about righting the plane from an in flight upset, and is therefore based on a disaster scenario, the message still applies to everyday flight: asymetrical thrust is not the preferred method of control, as the delay between adjusting the throttle and the actual thrust change makes it less precise for maneuvering than the main control surfaces.

Asymmetric thrust is sometimes used during strong crosswind takeoffs and landings to provide additional yaw authority for the airplane.  A pilot can set a difference in thrust between both engines to alleviate rudder pressure during said operations.  This is generally advisable only in steady crosswind conditions as gusts can make the control of the aircraft treacherous.


text:
Will occupants usually survive a decent autorotation?
When I learned to fly helicopters, I of course spent significant time learning about and practicing autorotations.
The CFI at my school, who had around 15,000 hrs (that's right, fifteen thousand!) said a few times that practice, knowledge and currency are vital — but as long as you got the entry right (following which you can fly to the ground) and executed at least a decent attempt at the flare and cushion, you would survive. Is this a correct take on the survivability of an autorotation? Do fatalities arise primarily from errors during the autorotation entry or flare?
While you might not get to use the machine again, and you might spend some time in hospital, you would live to fly another day.  I am assuming a reasonable place on dry land is available to finally come to rest and no post-crash fire.

To me, it's not entirely clear what you're asking. But I'm not a helicopter pilot so forgive me if it's obvious.

Entry is a rapid set of control inputs (lever down, stick back, pedal left (depends on which way the rotor turns) to maintain the attitude and preserve/restore rotor RPM if the donkey goes on strike. In autorotation, you are heading down, and more quickly than usual, but the machine is completely controllable. You can extend the glide, reduce the glide, s-turn, circle and so on hoping to alight on something friendly like a clear field. The bit at the bottom is a lot more complicated but in essence converts the energy of falling into lift to cushion yourself to a smooth touchdown.

Yes, I know what autorotation is, but your question isn't very clear IMHO. What exactly are you asking?

@PhilippeLeybaert Ah, sorry, I see what you mean.  I have edited the question to hopefully make it more clear.  I am interested in other opinions.  I guess another way of stating it is "does an auto need to be more or less perfect at the bottom to survive?"

@Simon - I tried to clarify the question a bit further. If this isn't what you wanted to say please revert my changes!

Putting the one-sentence "elevator question" in bold is always a good idea.

@egid.  Thanks!  Why didn't I do that ;)

If the pilot flubs the flare and landing, the result is frequently survivable.  The reason is that even a flubbed autorotation will reduce the vertical speed.
Student and instructor walked away from this one:
http://www.nydailynews.com/news/national/helicopter-crashes-westchester-county-airport-article-1.1560222

Thanks Skip.  Do you happen to know if that was at the bottom of an auto or something (slightly) less dramatic like a dynamic rollover?

I have no direct knowledge, but the news implied that it was the bottom of an auto.  Also, the flight school on field does practice autos in that location very frequently, so my assumption is that this recovery was a little too late.


text:
Should you read back the QNH from an ATIS broadcast?
A typical ATIS broadcast includes the QNH ("altimeter" for the cousins).
My habit is to read this back since I have effectively been given the setting by ATC and it is required for my startup. For example:

Finden 37 with information Delta, the QNH 1022.

I can't find any reference on whether this is required or not.  Additionally, do any CPLs or ATCOs on here consider this to be good or bad practice?
If someone wants me to tag with a jurisdiction, then it would be UK CAA since that is where I fly but I am interested in all responses.

Typically here in the US you just say "With Delta" and ATC assumes you have copied (and understand) all the ATIS information. Sometimes ATC will give an updated wind between ATIS, but the baro usually doesn't change that fast (hourly).

Uk CAA CAP493 Section 3 Chapter 1 Paragraph 11.4 states :

In the case of a departure ATIS, controllers must obtain a read-back of all relevant altimeter settings contained in the broadcast, unless the setting will also be passed in association with start-up or taxi clearance.

So from the point of view of the controller you are doing a good thing because you are saving him the expense of having to confirm that you have the right QNH on board.

ICAO Doc 4444 Section 4.5.7.5.1 states:

The flight crew shall read back to the air traffic
  controller safety-related parts of ATC clearances and instructions
  which are transmitted by voice. The following items shall
  always be read back:
  ...
c) runway-in-use, altimeter settings, SSR codes, level
  instructions, heading and speed instructions and,
  whether issued by the controller or contained in automatic
  terminal information service (ATIS) broadcasts,
  transition levels.

Can you link to this document? What is the jurisdiction?

google icao doc 4444. In uk the corresponding document should be CAP493 .

I think you mean section 4.5.7.5.1(c)...

By the way, it looks like the 4444 is a document for ATC controllers, not pilots. It says the controller must read back the setting, and the section you are quoting is about en-route flight levels.

I think flight crew is meant to be understood as pilots. I was pointing you to pararaph c) " runway-in-use, altimeter settings, SSR codes, level instructions, heading and speed instructions and, whether issued by the controller or contained in auto- matic terminal information service (ATIS) broadcasts, transition levels."

@RonBeyer  That document does require a readback of altimeter settings obtained from ATIS.

@Simon I am inclined to disagree. A plain reading of the text would indicate that only transition levels broadcast by ATIS need be readback, not altimeter settings.


text:
Which maneuvering speeds are included in the TCDS?
I've been looking at the data found in various type certificate data sheets and found something that is a bit ambiguous. In reading a TCDS (like this one, for example) in the "airspeed limits" section it lists the maneuvering speed. But they don't make it clear whether they are referencing the design maneuvering speed (Va) or the Maximum operating maneuvering speed (Vo).
Can anyone clarify which one they are referencing?

The speeds that they list in your linked TCDS (along with their corresponding V speeds) are:

Never exceed 171 mph (148 knots) CAS - VNE
Maximum structural cruising 140 mph (121 knots) CAS - VNO
Maneuvering 129 mph (112 knots) CAS - VA
Flaps Extended 115 mph (100 knots) CAS VFE

For a list of these V Speeds along with their definitions, see 14 CFR 1.2


text:
Is there a formula to compute the elevator angle to stabilize takeoff?
I'm building a ground effect vehicle, but I'm failing to stabilize the lift-off. At a certain speed, the lift force is equal to the weight force, so the vehicle lifts off. But this implements a change in angle of attack, so also an uncontrolled change in lift, as can be seen in this video.
I know that I can solve this with an elevator that can oppose the change in angle of attack. Now I'm searching for an equation to calculate the elevator angle to get a stabile and controllable lift off.

I think your problem lies in the CoM placement compared to where the CoL is, each time you get off the ground the vehicle pitches back dramatically and stalls (or that is just the drag of the tail section)

I would approach this problem a little differently.  Think about calculating the amount of lift that the elevator is producing given its angle and speed, and go from there.

You need more than the correct elevator incidence. Natural longitudinal stability works by distributing lift over the flow direction such that forward surfaces create proportionally more lift than rear surfaces.
Now it is important to know that only pure supersonic lift will be evenly distributed over the chord length of a wing. In subsonic flow the center of lift is at 25% (and even less for wings of small aspect ratio), so you now need to place the center of gravity such that it will be ahead of the 25% chord line of the sum of all lifting surfaces. To distribute lift for this forward center of gravity location, the incidence of the elevator needs to be several degrees smaller that that of the wing. Adjust incidences (say 5° for the wing and 0° for the tail), and then trim the center of gravity by adding ballast or shifting masses to make the aircraft fly straight.
Since airfoil camber shifts the incidence of zero lift to more negative values, you can alternatively use a cambered airfoil for the wing (say, at now 2°) and use a symmetric airfoil for the elevator at again 0° incidence.

tl;dr, adjust center of mass forward

@ratchetfreak: Right, but I hope they now understand why.


text:
What is the reference length in calculating the friction coefficient?
I am building a ground effect vehicle and I want to calculate the parasite drag of it.
So I found this site to calculate the friction
But I don't know what the "reference length" is. Is it the chord, the wingspan or the mean aerodynamic chord? And in case that the reference length is the chord, is their a different way to measure it for an inverted delta wing?

Since the site computes atmospheric conditions, only the friction coefficient $c_f$ needs a reference length. This is the length of the object causing friction, measured along the direction of airflow. In case of a wing, it is its local chord, in case of a fuselage, it's the fuselage's length.
Each part of an airplane and each section of the wing will have a slightly different $c_f$, depending on local chord length. The highest contribution to friction drag is caused at the leading edge, when the boundary layer is thin and the speed gradient at the surface is steep. The farther downstream you look, the less each additional unit of length will add to friction drag, so the average over the full length (that is what $c_f$ stands for) will become smaller for longer bodies.
To compute total drag, you need to add the contribution of each part, multiplied with its surface area relative to the airplane's total area. Yes, for a wing the upper and the lower wetted surface area will each contribute their own drag component. Since airflow is faster on the upper surface of a lift-creating wing, you would need a correction factor to account for the speed difference between upper and lower side, but if the difference is small, you can just assume the same speed everywhere. To be precise: $c_f$ is the friction coefficient of a flat plate parallel to the airstream.


text:
How do aircraft refueling in-air deal with wake turbulence?
Many fighter planes have the capability to refuel in air. When a plane moves in air, turbulence is generated at its back. To refuel, the fighter plane must come near to the parent plane (which has fuel in it).
When fighter plane reaches the back of parent plane, it is actually in the turbulent zone of the parent plane. How does fighter plane manage to fly and precisely adjust its position even in the zone of turbulence?

The receiving plane is quite a bit below the tanker. I'd think it's enough to be below the wake vortex.

@JanHudec - they avoid some of it, but still catch a good bit. And the boom itself produces some turbulence that makes the last few feet rather difficult

Jetstream is definitely worth a watch. It shows quite a bit of refuelling training.

I'm also thinking that wake turbulence at altitude is not as dangerous as near the ground.  If a jet were to flip over, they would be able to recover safely.

Ex US Navy pilot here.  I flew the S-3 Viking, which refueled via the probe/drogue method, as opposed to the USAF flying boom method.  I can confirm that the receiving plane is below the wake turbulence of the tanker, maybe by 8–10 feet.  Although, even if you should ride up into the wake, the sensation is perhaps not as violent as you might think.
The technique they teach is that you just fly formation.  You do NOT fixate on the basket (this is the 'drogue' part of the apparatus, which resembles a big badminton birdie).
Instead, you must position the tip of your probe about 4 feet aft of the basket, stabilize, then smoothly add throttle until contact is made.  You may need to hit the basket at a couple of feet per second relative velocity in order for it to latch.
Then you have to maintain position.  It's just formation flying, really.    The hose apparatus on the tanker measures your position fore and aft – when you are within limits for fuel transfer, a light goes on.  So, as the receiving pilot, you fly formation and maintain your position fore and aft so that the light stays lit.  When you've taken on your gas, you need to exit straight back.  Too much vertical offset can cause the basket to 'whip' as it disconnects.  You can smash a canopy this way – I've seen it happen, and it's pretty colorful.  You won't soon  live it down.

Former B-52 crewdog here...The receiver aircraft flies below the wake turbulence of the tanker.  In the case of the typical USAF method, using the boom, the receiver aircraft flies into a "contact" position and holds there.  The boom operator (boomer) then "flies" to boom to the correct final position, then extends the nozzle into the refueling receptacle, the receiver aircraft just has to stay in the correct position relative to the tanker.
Whereas, as with "probe and drogue", the shuttlecock basket is trailing behind and below the tanker, the receiver pilot flies their aircraft such that the refueling nozzle goes into the basket.  I'm not sure if the tanker has any directional control over where the basket goes other than just extending the hose.
Although the USAF primarily uses the rigid boom method, there were some number of KC-135R's that had the drogue on the end of the boom, so they could refuel Navy and some NATO aircraft.  I don't know about the KC-10, but I would imagine there a few of them also with this capability.

According to Wikipedia, the KC-10 doesn't need that capability, as it has (depending on its place in the production sequence) either one or three dedicated drogue pods in addition to the boom.

Like to see the fleet and crews talking here. I am a US Navy A7E pilot who flew in the Med aboard the USS Nimitz in the mid-1980's. I served as tanker pilot, and had my share of plugging behind an A6 or A7 at night. Hit a KC-135 once with a flyable boom and that was sweet.
If you are tanking off of something like an A6, which I believe was designed with a refueling mission in mind, it isn't to bad. There isn't too much  turbulence behind them. The store is internal to the fuselage and the drogue extends directly behind. Now the A7E is a different matter. Sitting behind it, with its "short" hose length, your vertical stabilizer stuck right up into the tankers jet wash. It was a rough session getting bounced around, but it is random and so doesn't affect you too much, i.e. it doesn't move the probe around much. I remember in particular random yaw inputs here and there, but one was much more susceptible to pilot induced oscillations than this sort of nuisance.
I had one of those nights where my first approach wasn't, let's say, the best, and I boltered off the end of the deck. As I entered the blackness in front of the ship I was praying that the controller turned me down wind. I could imagine my skipper in Ops saying, "Give him another chance at the deck ..." Instead I heard, "Alpha Juliet 4-2-7 what is your fuel state?" I replied that I was at 2,000 pounds. 1,200 pounds was emergency fuel for us and in the pattern you were at 3,000 to 5,000 pounds per hour consumption. With 800 pounds you have around 12 minutes flight time until things get really serious. Next was the call, "4-2-7 your signal is tank, tanker 9 o'clock, 5,000 feet."
I joined on the tanker, an A7E flown by my squadron mate "Boris". I was stationed alongside when he told me the drogue was extending. He carried a buddy store (tank) under his wing with 2,000 pounds of gas to give away. He had been directed to give me a 1,000 pounds. When the hose had successfully extended he cleared me in to tank. I moved back and got in position feeling the familiar jet wash hitting my tail. It was a very black night. The drogues at the end of the hose are not that big, kind of like the size of a basket used to carry apples in the orchard. It had lights all around the circumference to help you locate it and give you some depth perception. Of course in operation the lights seldom worked. You relied on the probe light on your aircraft that came on when you extended your probe and it is like a headlight on your car, but not that bright.
The A7E was a bit peculiar when it came to refueling, in that the probe extended out of the right side of the fuselage alongside you in the cockpit. The probe wasn't much in front of you. So you were steering the probe in to the basket while it was off-center. Not a big deal, but it did get time to get used to. I was behind him and stabilized and working my way up to the basket. You fly the tanker, keeping the basket in your peripheral. The minute you focused too much attention on the drogue probe closure, and this would usually happen for me when I was close to plugging in, the pilot induced oscillation would start. Trying to correct for a small difference leads to a bigger, leading to a bigger, and so on. You get the picture. Ugly.
As I came in to plug I was nervous and gripping the stick tightly, never a good sign, I was barely holding it together when I got the probe deep in the basket, but hit off center with too much closure, bowing the hose. Then the oscillations started and that was that. I was trying to recover and eventually slammed the throttle to idle to get out the drogue as quick as possible. This situation can lead to ripping the hose out of the buddy store, which then can lead to a fire on the tanker. There was this big bow in the hose, and I watched the tip of the probe slip off the basket. The basket, now in the tankers wind stream, whipped around, coming back, slamming into the canopy. Oh, yeah that is the other thing that can happen. In my case the canopy did not break and I was relieved. I still had to get my fuel and it took a few more attempts, all better than the last. At some point Boris asked me if I was alright back there. "Oh yeah, no problems back here. There finished." When I left the tanker I was asked my fuel state, and when I looked down I saw it was at 1,900 pounds. I had spent all the fuel he gave me trying to get fuel. "Ah ... right-o ... 2,500 pounds."
Here was the other thing that they didn't tell you about getting your canopy slapped by the drogue. It sprays the fuel all over your canopy, which in close to the deck, turns into the most beautiful rainbow streaks making it nearly impossible to see the ball.

Practice. Lots of practice.
Smaller planes like fighters usually refuel from a hose trailing behind the wing, far enough from (and below) the main body to avoid the worst turbulence. The tanker is completely passive here - the receiving pilot does everything. If it's too turbulent he just backs off and tries again
Larger planes use the flying boom method - they're the ones who have to come in right behind the tanker and hold position while the boom gets slotted into place. These planes are a lot heavier than a single-seat fighter and can handle the turbulence better. That doesn't mean it's easy - there are plenty of near-miss videos on YouTube.

Actually, this isn't quite right. Large and small aircraft can use either method, it depends on the service. USAF aircraft (except helicopters) use the flying boom method, while the USN and marine corps, and USAF helicopters use probe and drogue methods.

@GdD it really depends on the receiving valve on the plane; the boom and drogue line are incompatible

Absolutely true, an aircraft has to be ordered with one option or the other, or have one or the other retrofitted.

The problem is less turbulence than downwash. Flying behind and below the tanker means that both vertical and horizontal position will change the downwash in a way that makes holding the position difficult. Flying behind a tanker is flying in an unstable equilibrium.
Both falling back and climbing up will position the receiving aircraft in stronger downwash. If the receiving aircraft moves forward relative to the tanker, it flies into reduced downwash angles which let it accelerate further, because with less downwash less power is required for the same flight speed. The opposite happens when it falls back, now the pilot needs to advance the throttle to keep his position.


text:
What is the weight difference between 100 hp and 150 hp Continental engine?
What would be the approximate difference in weight for a Continental 100 hp engine compared to a 150 hp?

Which variants are you specifically talking about?
The Continental O-200-A (100hp) weighs 170.18 lb (77.19 kg) dry, without accessories.
I can't find a 150hp Continental, so here is the closest variant I could find:

Continental O-300 (145hp) weighs 268 lbs (121.5 kg) dry, without starter or generator
The O-300 is also listed as a C-145 and as a C-175
E165-2 165 hp dry weight 351 lb (159 kg) - Variant of the O-470
E165-3 165 hp  dry weight 352 lb (160 kg) - Varian of the O-470

I hope that's what you were looking for.


text:
Can a fighter jet land on a modified airliner?
Imagine this scenario:
A modified military 747 is flying at a constant speed and altitude. On the top of the fuselage there is a device like a helipad but with three holes to accommodate the fighter jet landing gear, when pressure is applied the holes close, locking the landing gear in the holes. The fighter is flying at the same speed and slightly above. With proper training, is it possible for the pilot to land on the plane and then shut off the engines?

Technically yes. But I am unaware of any attempt to do this with airliners.
But there were several designs in the past which used a big airplane or a Zeppelin as a mother ship, which took fighters with it for air defense. The B-36 was involved in several such designs.

In all cases the docking was made from below, because this gave the fighter pilot the best field of view and reduced the risk of a tail strike.
While the fighter in the picture above is a Republic F-84, McDonnell even designed one specifically for the purpose of being carried around by a bomber, the XF-85 Goblin. It used a rather complicated trapeze for docking and was then pulled up into the fuselage, as the picture with a B-29 below shows.

Your design proposal with the simple platform on top of the fuselage would carry a high risk of a tail strike if anything goes wrong, so it is unlikely that this will ever be tried for real. But when you change the airliner to a AN-225, the idea looks not totally impossible. The Russians tried to dock a fighter with a mothership as early as the 1930s. The picture below shows a TB-3 docking with a I-Z fighter, the first pair of aircraft in history to dock in flight.

The main reason for docking was the much smaller range of fighters. Today, air refuelling takes care of this deficit, so it is unlikely that anyone will see a benefit strong enough to justify the expense of modifying two aircraft and the risk incurred in the docking process.

This seems like a Darpa-hard problem. Certainly it would have its challenges, but I suspect with computer control of the aircraft it could be done. I would not expect a human to be able to do this safely.

@rmayer06 70 years ago "how to get fighter support at the end of really long bombing missions" was a Darpa(predecessor)-hard problem.  Having fighters capable of docking with and being carried by a bomber was an idea that they tried; but ultimately rejected because aerial refueling turned out to work better with less additional pilot skill required and lower risk of catastrophe.

@theMayer, any pilot skilled enough to fly in close formation ought to be capable of “landing” on another moving aircraft.  Computers aren’t the solution to every challenge...

Parasite jet aircraft have been experimented with, the only purpose built one I know of is the XF-85 Goblin, there was also a modified F-84 Thunderjet which was used as well. These experiments were abandoned because the process of launching and recovery was too hazardous, and air to air refueling made much more sense to extend the range of fighters.
It is possible to join 2 aircraft in the manner you describe, however it's extremely dangerous due to the aerodynamic forces involved, and there are no benefits which would make the risks worthwhile.

DARPA just happens to be asking for proposals on how to accomplish this very task:

DARPA invites input on how to enable existing large aircraft to carry,
  launch and recover multiple unmanned air systems for a variety of
  missions

Not quite the same task: a fighter jet, at the moment, is not an "unmanned air system".

@DavidRicherby: it is if the pilot isn't male. :-)

@BobJarvis: No, not really. "Manned" does not mean "crewed by a male". In fact, "man" doesn't even mean "male". That's a myth spread by feminists who do not understand the genericity of the term "man". The male equivalent to "woman" is "wereman". http://www.todayifoundout.com/index.php/2010/08/the-word-man-was-originally-gender-neutral/ et al

@LightnessRacesinOrbit - really? Wow - and here all these years I thought "wereman" meant "a wolf that turns into a human when the moon is full". Thanks, man!

@BobJarvis: :DD

@Lightness, you missed a key word in the headline of that link: "The Word ‘Man’ was Originally Gender Neutral". Now it means exactly what everyone (except you, apparently) thinks it means.

"[T]he word ‘man’ was originally gender neutral, meaning more or less the same as the modern day word “person”.  It wasn’t until about a thousand years ago that the word “man” started to refer to a male and it wasn’t until the late 20th century that it was almost exclusively used to refer to males."

@Reid: So you think "mankind" is men only? Grow up.

I'm not going further down a Gish Gallop of strange, misogynistic theories unsupported by the very evidence cited.

@LightnessRacesinOrbit: right. Little-known fact, there's a prophecy in LoTR, that the Witch King can't be killed by a man. He's killed by a woman. Only feminists are able to understand the plot point, since to everyone else the prophecy unarguably states that he can't be killed by a werman or wifman, whereas feminists subscribe to Tolkien's myth that the word "man" has unclear or ambiguous meaning ;-) In real life, I think you're correct to say that "manned doesn't mean crewed by a male", but incorrect to say "man doesn't even mean male", since often it does.

@SteveJessop ;)

This went in an unexpected direction.

It is not possible to replicate this process safely.  The pilot in the smaller aircraft can not see its gear and would have no idea where to  "land".  And perhaps even more importantly, there's no out if the fighter has an emergency.  If the fighter loses control just above the carrier then its likely that both aircrews will die.
As has been pointed out in other posts, aircraft have been recovered underneath carrier airplanes in the past, and while dangerous, this leaves an obvious escape route in the event of an emergency.

To enable a landing on top, modern technology can improve the view, see for example the "transparent hood" concept. Also, the carrier aircraft should actively catch the gear and pull the fighter in. But still, a tail strike is a real risk.

@PeterKämpf And what happens when those systems fail? Even when they work, there still isn't any depth perception, it would be very hard to judge your height above the carrier, even if you could see through the aircraft. Not to mention, its an insanely bad idea to fly form belly up to another aircraft. And again, emergencies become fatal. There is no benefit to landing above and it could not be done safely. Note: doing it once to prove a concept is not a measure of safety.

I never said it's a good idea. I agree with your verdict.

As an addition to the projects described by Peter Kämpf, there was the Tip Tow project during the fifties where two fighters were attached to the wingtips of a B-29. The idea was that the additional wingspan would increase the range of the bomber, while at the same time enable it to carry its own fighter escort which could attach and detach in the air as required. While attached the fighters could switch off their engines, thus saving fuel for later.
While attached, the B-29 would control the flight surfaces of the attached F-84 (more specifically EF-84D) so that the pilot wouldn't have to do this manually, but during the first flight with this system enabled one of the fighters rolled on top of the B-29s wing, and the two aircraft crashed with no survivors. The second F-84 survived as it was disconnected from the B-29.

Don't forget the good old space shuttle:

Although the shuttle was not landed on the 747, it does demonstrate that it is aerodynamically possible to fly piggyback.
The picture illustrates another challenge of trying to land on the back of another plane - you have to clear or otherwise avoid the rudder. And once it landed, the piggyback plane would be blocking the airflow to the rudder - which is why they added those vertical fins to the elevators.
So ideally, the mothership would be designed with twin rudders that are not on the centerline - like on the old Connies (but without the center rudder).

Or like the An-225, which was designed to carry a space shuttle.

authors: Bob Jarvis - Слава Україні


text:
Sell 1/4 share or buy 3/4 share and leaseback?
I currently own a 1/4 share of an Archer, but due to relocating I have put it up for sale and was planning to find another partnership once its sold. It's been on the market for about 3 months and have not had many people contact me with interest (no one serious). The other option I am debating is to buy up the other 3 shares and make the plane available for leaseback down in my new area.
Anyone have any feedback if leaseback is a good option? If I can get 80hrs/month (which is what the FBO owner says he's getting on each of his 172s), then I'd earn enough to cover the loan for buying the 3/4 shares, but I'm very concerned of hidden pitfalls. I've seen the condition of most rental planes & I'd be appalled if my Archer ever looked like those. Does anyone have any advice/suggestions?

The cost/benefit tradeoff with leasebacks is VERY complex - I don't think we can really advise you well in the Q&A format of the main site, but if you want to stop by chat it would be a good topic for a discussion.


text:
How does cost per seat-mile differ between commercial and private jets?
I was reading an article the other day that mentioned many smaller private jets are more efficient, faster, can climb quicker and land with less space than larger commercial aircraft. Some of the obvious metrics make sense such as shorter landing distances, but the efficiency peaked my curiosity.
I am basically asking what is the cost difference or cost per seat mile difference in say a a Boeing 737 vs a smaller often private jet, say an Embraer Legacy 600 or similar with its maximum seat capacity setup.

They've been making 737's since 1966 and efficiency has changed dramatically between the various generations. If you're referring to current generation 737's, you may want to be more specific. Not all 737's are the same size either.

AirAsia has the lowest unit cost of any airline, at US$0.023 per available seat kilometers (ASK) and a passenger break-even load factor of 52%. That's on a fleet of mainly A320-200s.

Regarding private jets, here's an article about the fare price comparison of a private jet vs easyJet - it looks like only when its on an 'empty leg' that the private jet can be fare price competitive to the customer.

Are you referring to the maximum seats that would be allowed for the standard "private jet" interior, or how many that jet would fit in an airline-style configuration?

I was referring to a comfortable but max amount of seats in a private jet ie. no couches/bars etc seen on celebrity planes.

I guess I should/can specify more. If the same company owned a remotely modern 737 say one that AA is currently flying from the 90s-00s I guess and also owned a Legacy 600 or similar what would be the cost per seat be for each after fuel, staff etc assuming normal conditions

I did some basic math to figure it out in terms of current gas prices.  I got that a private jet is about 6x more expensive.  Keep in mind it is probably more in reality because these jets aren't being used as much as an airliner so the rental price will be higher as well as the fuel.  But like I said, this was just an estimate based on current fuel prices and performance of an airliner vs a private jet, to give you an idea of the cost difference.

Good illustration right here.  It's basically like comparing private cars to mass transit.  Mass transit always wins in efficiency, but not always (or even usually) in convenience.

By the way, that fuel price for the airlines is WAY too high, and even private jets usually buy contract fuel and get fuel at cheaper locations than LAX.  Airlines buy is such bulk though that they won't be paying anywhere near that, which gives them an even bigger advantage.

Thank you. That is what I was asking for. Since I don't know were you got the gal/min cost from, if we changed it to 14 passengers and 600mph what would that be? Not concerned with the accuracy of gas price at the moment this is more for comparison at any gas price.

@Lnafziger if any thing, airlines should be able to get fuel cheaper due to futures contracts and economies of scale...

@robokaren That's what I was saying in my comment!  :-)

@Shaun Well, just for the record, there will normally be a rather large price disparity between fuel prices for the airline versus private jets that will skew the numbers even further towards the airliner.

I think you're forgetting some other important operational costs.  Like the ramps fees and the pilots salary.

@JayCarr, this was just to get the basic concept on paper (which I think it does).  Obviously there's a WHOLE lot more to it.

I understand where you're coming from, but I think that other costs differences between large and small planes would have a very significant impact on the answer. I guess what I'm saying is I feel this is over simplified.  Though I would agree that it's a reasonable starting point.

Nicely done. Q.E.D.


text:
Where can I get IFR charts for throughout the world?
(Note this question asks about VFR charts.)
In the U.S, I can get IFR navigation charts from Aeronav or Jeppesen.
Do IFR charts exist worldwide? If so, where can I go for charts of other countries? (A list would be nice since there probably isn't one canonical source?). If not, what are the alternatives?

The primary source for every country would be the countries' Aeronautical Information Publication (AIP). In some cases the AIP is freely accessible, printed copies can be directly obtained from the Civil Aviation Authority (CAA) or commercial companies authorized to distribute these, such as Jeppesen.
In some cases there will be a joint resource for certain areas, e.g.
The IFR charts for European countries can be obtained as digital copies from EAD EUROCONTROL.

https://www.eurocontrol.int/articles/ais-online This link is a good source for finding AIS information for almost the entire world

Not sure whether you want them just for informational purposes or for actual flight planning.
https://skyvector.com/ has IFR and VFR charts (and airport information) for the entire planet. I don't know how current they are and whether they are legal for use as operational charts for aircraft operations, but for informational and simulation purposes they're certainly good enough.


text:
How do airlines assign flight numbers?
(Not company call-signs)
Wikipedia gives the following guidelines

Eastbound and northbound flights are traditionally assigned even numbers, while westbound and southbound flights have odd numbers.
Other airlines will use an odd number for an outbound flight and use the next even number for the reverse inbound flight.
For destinations served by multiple flights per day, numbers tend to increase during the day.
Flight numbers of less than three digits are often assigned to long-haul or otherwise premium flights.
Flight number 1 is often used for an airline's "flagship" service.
Four-digit numbers in the range 1000 to 4999 typically represent regional affiliate flights.

but doesn't give any references (anybody have any?).
Assuming these are (generally) true, are there common strategies for how airlines number flights in a 'class'? For example, is there any significance between DAL310 and DAL342? AAL4569 and AAL4135?
Or is the standard method generating a (bounded) random number?

Link 1Link 2 Here are 2 links I found for you about flight numbers

This is going to be specific to each airline. They can assign flight numbers arbitrarily and they can all do it differently.

Highly specific to each airline. There's no general rule. Question has no answer sorry.


text:
When is a control tower required?
I know there are busy airports without control towers, and the converse is also true. There is a process for an airport to acquire a control tower. Sometimes, airports want to have their own control tower.
Is there some activity level where an airport would require a control tower for safe operation? Or does air traffic control exist simply to reduce risk?

As far as I'm aware there's no specific regulation that requires a control tower to be established. FAR 170 and the regs referenced therein set out when an airport becomes eligible for a tower, but nothing requires an eligible field to have a control tower (in fact the FAA won't establish a tower unless someone gives them the land).
That said it would probably be a Bad Ideatm for certain airports to operate without a control tower (Atlanta, Kennedy, O'Hare, LAX...) - in fact those airports are busy enough that they have additional traffic management procedures in place beyond what a control tower can provide.

Control towers exist to improve safety (improve risk) as you alluded to. There are procedures and rules established to allow pilots to self-sequence at untowered fields (you'll find those in section 4-1-9 of the AIM), and they work very well when everyone follows them.
The catch is everybody doesn't follow those procedures - or more accurately they're followed with varying degrees of precision (my personal favorite example of sloppy procedure is when you hear someone say "any traffic in the area please advise" at an uncontrolled field). These procedures also don't cover other movements on the airport surface very well: Some pilots will announce their taxi plan at an uncontrolled field, but there are plenty of pilots who taxi from point A to point B with their head up and their mouth shut so as not to tie up the frequency for aircraft in the air.
As the number of operations increases at an airport the various degrees of precision and adherence to the uncontrolled-field procedures in the AIM  becomes more of an issue, and a control tower may be established by the FAA or the local airport authority to safely and efficiently coordinate aircraft movements.

Wouldn't the FAA likely deny an airport certificate to a big airport with no tower?

@cpast A small airport can have more traffic than a large one, or an airport could start out small and grow (in both traffic and land use). That's why the standards for establishing a tower are mainly traffic-volume based. FAR 139 doesn't actually require a tower as best I can determine though there's probably a catch-all that could prevent you from getting on a chart...

By "big" I meant "busy." And an airport serving scheduled passenger traffic on not-small airplanes needs Part 139 certification and a certified operations manual, which can be ordered amended by the FAA's regional manager (the rules seem to have a catch-all if "safety in air transportation requires it"). By the time an airport gets big, I imagine the certification manual would be ordered amended to deal with heavy air traffic (i.e. using a control tower).